Carbon materials, generally referred to as activated carbons, for adsorption, liquid cleanup, gas cleanup, gas storage, and monolith structures are widely available from many sources. Useful carbon materials have high surface areas and a high density of pores with optimal diameters. Table 1 lists the diameters considered to be critical (i.e., pore diameters below which the molecule would not fit into the pore) for adsorption. Observations and theory tend to agree that the optimal diameter for adsorbing a molecule is about 2.7 times the critical diameter, with optimal pore diameters of 6 Å, 6 Å, and 11 Å for hydrogen, acetylene, and methane, respectively.
TABLE 1Common molecules and their critical diameters (Dcrit)aMoleculeDcrit (Å)Hydrogen2.4Acetylene2.4Oxygen2.8Nitrogen3.0Water3.2Methane4.0Methanol4.4aMineral Adsorbents, Filter Agents and Drying Agents. Aldrich Technical Bulletin. http://www.sigmaaldrich.com/Brands/Aldrich/Tech_Bulletins/AL_143/Molecular_Sieves.html.
The available carbons, whether derived from fossil fuels or biomass, rarely have surface areas in excess of 2000 m2/g and generally have pore diameters and pore volumes such that they are not able to adsorb and store >20% of their weight in natural gas (methane) at ambient temperature and a pressure of 500 psig. Thus, there is a need for a carbon material, preferably derived from biomass and hence renewable, with a high surface area and a high volume of pores with diameters in a range that promotes high storage capacity of natural gas and other energy carriers. Activated carbons having these properties would be useful in a wide range of applications, such as fuel tanks in vehicles, batteries, electrical capacitors, separation and purification devices, and catalysts.